The influence of visual deprivation on the neural network for language : PhD thesis

Dzięgiel-Fivet, Gabriela

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The influence of visual deprivation on the neural network for language : PhD thesis

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Title: The influence of visual deprivation on the neural network for language : PhD thesis

Abstract:

The functional organisation of the human brain is influenced both by innate mechanisms and individual experience. Spoken language processing, an evolutionary old skill, occurs in a neural network universal for different languages. On the other hand, reading is a skill that appeared in human evolution quite late and thus is an excellent example of neural plasticity connected to learning a new skill. Additionally, reading can be performed using not only vision but also touch. Braille alphabet is a script used by the blind population for reading using the sense of touch. Blindness enables us to see which aspects of the neuronal organisation are fixed and which change with altered experience.The current thesis focuses on the plastic changes in the organisation of the neural language network following visual deprivation. Three studies were conducted. The first focused on mapping the spoken and reading neural networks in the blind population and comparing them to the organisation of language processing in the sighted. Speech-reading convergence – a phenomenon thought to be universal in print reading was also tested for the first time in the blind population. The results of Study 1 revealed that speech-reading convergence was present in the blind subjects, but in different areas. It was found in the ventral occipitotemporal cortex (vOT), instead of the perisylvian regions. In the blind group, the vOT was active not only during reading, as in the sighted, but also during speech processing. The temporal cortex, which is involved in phonological processing in the sighted population, was disengaged during Braille reading.Thus, in Study 2, the vOT engagement in phonological processing was studied in the blind and the sighted. The blind subjects activated the left vOT during auditory phonological processing to a larger extent than the sighted subjects. However, this activation seemed not to be phonology specific. In the blind, the left vOT presented a similar activation during linguistic processing as other regions of the language network. The results of the second experiment suggest that the vOT plays a more general role in language processing in the blind population due to changed input to this structure arising from visual deprivation.Study 3 tested the differences in the cognitive correlates of print and Braille reading. Additionally, the relationship between literacy-related skills and age was studied using a cross-sectional design. The results of the third experiment indicate that the change in the modality used for reading introduces some alterations to the cognitive mechanisms of reading. Limits of the tactile modality - lower processing speed and the sequential nature of the processing augment the importance of the haptic factors for Braille reading and may cause minor deficits in some domains. On the other hand, different demands induced by the changed modality strengthen phonological skills and short-term memory. Yet, the developmental trajectory of literacy skills remains unchanged in the blind, as there were no differences in the correlations with age between the groups.Research presented in the thesis demonstrates that visual deprivation influences the functional organisation of both evolutionary old (spoken language) and newly learned skills (reading) on the neural and behavioural levels. Results underline the importance of individual experience for the organisation of specialised neural networks and are in line with the pluripotent cortex hypothesis of neural plasticity